Improve MetaSSR

MetaSSR already delivers fast SSR and does well on the benchmarks, which is promising. But the dev mode, tests, and error handling still need hardening to be production‑ready. This project aims to make MetaSSR more mature and production-ready, focusing on improving the API handler (the core idea of MetaSSR, Polyglot programming via MetaCall), stabilizing and adding feature to the dev mode, expanding tests, and improving CI/reporting and documentation. If needed, the work will include changes to the MetaCall Rust port to support MetaSSR reliability & maintainability.